Fix first: upgrade to Prefect >= 3.2.14. The flow loader was not thread-safe in older versions; PR #16458 (v3.1.9) added a threading lock and unique module names, and PR #17524 (v3.2.14) uses real script filenames as module names and keeps modules in sys.modules.

```text
ModuleNotFoundError when simultaneously running deployed flows using custom image
```

1. Upgrade with `pip install "prefect>=3.2.14"`.
2. Rebuild your custom image with the new Prefect version and redeploy. Expected: simultaneous flow runs each resolve their own imports, no more ModuleNotFoundError.

When to use: deployed Prefect flows crash with ModuleNotFoundError only when several run at the same time.

When not to use: a single flow run fails with ModuleNotFoundError (that is a missing package in your image, not this bug), or flows running on stock images.

Compatibility: Prefect >= 3.2.14 (partial fix in 3.1.9). Custom deployment images.

Variants:
- "prefect concurrent flows ModuleNotFoundError"
- "prefect custom image simultaneous runs import error"
- "prefect load_script_as_module thread safe"

Root cause: load_script_as_module was not thread-safe. It shared the '__prefect_loader__' module name, modified sys.path without locking, and evicted modules from sys.modules after loading, so concurrently loading flows interfered with each other's imports.

Edge cases: if you pinned an older Prefect for other reasons, 3.1.9 has the lock and unique names but not the real-filename module names. 3.2.14 is the fully fixed version.

---

## Vectle citations

- Exact version: <https://vectle.com/skills/skl__J2_ZrmMkiuu6DS9QQERfg/versions/skv_ek5cP7GboQ9UTFbsL4Xgsg>
- Canonical HTML skill page: <https://vectle.com/skills/skl__J2_ZrmMkiuu6DS9QQERfg>
- Current Markdown URL: <https://vectle.com/skills/skl__J2_ZrmMkiuu6DS9QQERfg/markdown>

## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://github.com/PrefectHQ/prefect/issues/8337"
  ]
}
```